[The effect of fresh gas flow on the minute volume of anesthesia ventilators with a gas reservoir].

The tidal volume (TV) delivered by conventional anesthesia ventilators is dependent on fresh gas flow rate (FGF). When FGF is reduced, the TV declines; this must be corrected by increasing the ventilator bellows excursion. In addition, the falling bellows produce a negative pressure during the expiratory phase, which may result in positive negative pressure ventilation (PNPV). We have measured the performance of three ventilators: AV 1 (Dräger), VIVOLEC (Hoyer), and ELSA (Engström) that are equipped with a reservoir bag supplied with fresh gas and from which the bellows is filled. METHOD. Two breathing bags with a corrugated tube of 1 m length were connected to a Y-piece to simulate clinical conditions. Starting from 10 l/min, FGF was decreased by 1 l at a time down to 1.0 l/min. Measurements were made at each level of FGF and also at 0.5, 0.3, and 0.2 l/min using a constant inspiration: expiration ratio of 1:2, displacement of the bellows of 700 ml/breath, and a rate of 10/min. Measurements of peak pressure, positive end-expiratory pressure (PEEP), and delivered TV were made at each FGF setting. RESULTS. The course of TV-dependence on FGF is shown in Fig. 1, that of peak inflation pressure and PEEP in Table 1. Reducing FGF had no effect on TV and inflation pressure with ELSA. VIVOLEC lost 17% of the initial TV when FGF was reduced to 0.2 l/min. By closing the relief valve, the loss of TV could be reduced to 4.5%. AV 1 lost about 10% of the initial TV when FGF was reduced to 0.5 l/min. Peak pressure and PEEP were lower with minimal flow. The reservoir bag collapsed when the plateau pressure exceeded 18-20 mbar. CONCLUSIONS. The TV delivered by the ventilators studied was found to reflect closely what had been preset on the bellows displacement scale, within a range of less than or equal to 17%, irrespective of FGF. Hence, the problem of FGF-dependence of TV is largely negated in anesthesia ventilators equipped with a reservoir bag. With ELSA, there was no loss of TV even under minimal flow conditions. VI-VOLEC (with the relief valve closed) and AV 1 had a loss of less than 10% of the initial TV. Adding a reservoir bag to anesthesia ventilators is an effective method of guaranteeing a TV independent of FGF. The three ventilators tested here proved suitable for minimal-flow anesthesia.